The present invention relates to a method of generating image scanning clock signals in an optical scanning apparatus.
There are known optical scanning apparatus in which a light beam is periodically deflected by a rotating light beam deflector such as a rotating polygonal mirror or a hologram grating disk for reading or writing information.
The light beam, such as a laser beam, is deflected by the rotating light beam deflector into a scanning beam which is focused as a spot on and scans a surface through a suitable optical system. The surface being scanned may be an original having information if such information is to be read, or may be a photoconductive photosensitive body when information is to be written. While the scanning beam is scanning the surface, the information is read or written at the rate of one pixel per one clock pulse in response to image scanning clock signals.
The scanning of an object surface with a scanning beam is known as "main scanning". During main scanning, the object surface is fed in a direction normal to the direction in which the main scanning occurs. The feeding of the object surface is called as "auxiliary scanning".
In order to read and write information properly, it is necessary to align main scanning starting points in the direction of the auxiliary scanning. If the main scanning starting points were not aligned in the auxiliary scanning direction, then an image reproduced from read-out signals would be distorted or an image written by the scanning beam would be subjected to distortion called "jitter".
It has been general practice to align main scanning starting points in the auxiliary scanning direction by providing a light sensor outside of a main scanning region near the main scanning starting points and detecting the scanning beam just before each main scanning cycle so as to synchronize the main scanning cycles (see, for example, Japanese Laid-Open Patent Publication No. 61-175611).
One way of synchronizing the main scanning cycles is to use image scanning clock signals. According to this process, the instant the light sensor detects the scanning beam, image scanning clock pulses start being counted by m, and a main scanning cycle is started when (m+1) pulses are counted. The image scanning clock signals are successively generated. Since the interval between output signals from the light sensor, which serve as a reference for synchronization of main scanning cycles, varies due for example to the mechanical accuracy of the rotating light beam deflector, the time interval from detection by the light sensor of the scanning beam to the starting of a main scanning cycle varies up to one clock pulse at maximum dependent on whether the light sensor output signals are generated upon high or low levels of the clock signals, resulting in scanning irregularities.
Variations in the time interval from detection by the light sensor of the scanning beam to the starting of a main scanning cycle up to one clock pulse at maximum, mean that the positions of the main scanning starting points vary up to one pixel. There is a method for reducing the maximum positional variation of the main scanning points to 1/N pixel (N is a natural number), and this method is called a 1/N method for convenience.
According to the 1/N method, n pulse signals C.sub.1 through Cn are produced by a shift register from reference clock signals of the same frequency as that of image scanning clock signals, the pulse signals C.sub.1 through C.sub.n being successively shifted in phase by a constant phase difference such that the phase difference between the pulse signals C.sub.1 and C.sub.n are smaller than one period of the image scanning clock signals, and one of the pulse signals C.sub.1 through C.sub.n is selected as an image scanning clock signal. With this method, poiitional variations of the main scanning starting points are below 1/N at maximum where N=n-1.
The conventional 1/N method has a problem in that corrective clock signals of an extremely high frequency are required as shift clock signals for the shift register. For example, if the frequency of reference clock signals, i.e., image scanning clock signals is 10 MHz with N=20, then the frequency of corrective clock signals should be 200 MHz. Therefore, the cost of a clock generator for generating such corrective clock signals is high, and so is the optical scanning apparatus.